The present invention relates to phase shifters, and more particularly to a digital phase shifter for precisely and continuously phase shifting a continuous wave (cw) signal output.
A straight forward method for constructing a subcarrier regenerator for television systems is shown in FIG. 1. A reference subcarrier is input to a phase detector. The output of the phase detector is used to control, via an error amplifier, a voltage controlled crystal oscillator (VCXO) at the subcarrier frequency. The output of the oscillator is fed back via a variable phase shifter to the phase detector, and also output via a buffer amplifier as a phase shifted subcarrier frequency. This closed loop method provides a constant amplitude sine wave that has been phase shifted from the external reference subcarrier. The phase-locked loop bandwidth need only be wide enough to allow the variable phase shift to appear instantaneous, and should be narrow enough to do some phase noise averaging on the input reference burst or subcarrier.
Traditionally the variable phase shifting has been implemented using an electromechanical phase shifter such as a goniometer with manual control. This requires the subcarrier to be cabled to and from the front panel of an instrument. For a precision phase shift a second, limited range electro-mechanical phase shifter is needed. This prior concept is expensive and requires careful calibration. There is no reasonable means of remote control or microprocessor interface possible.
What is desired is a digitally controlled precision phase shifter that can be controlled by a microprocessor with a resolution of 0.1 degrees or better, and which is reliable, self calibrated and inexpensive.